I feel numbness in hand and i have low calcium levels in the blood. how long does it take for calcium to work?

It depends. Intravenous administration of calcium would work immediately. Oral intake will take a few minutes to half hour.

Oral calcium supplement alone may not be enough if you are low in active vitamin D3. The treatment depends on the cause of low calcium (hypocalcemia). I wonder how you know you have low calcium and how low it is. Calcium level above 7 mEq/L rarely causes symptoms. You should go to ER for IV calcium if you feel numbness around your mouth or you have hand cramping.

Hand numbness may not have any relationship to your calcium level, as this could be nerve pinching such as carpal tunnel syndrome. You should have your vitamin D levels checked also, as has been mentioned. Perhaps all of this needs concerted attention from your doctor ASAP.